The epidermal growth factor family of transmembrane tyrosine kinase receptors, including EGFR (ErbB1 or HER1), ErbB2 (HER2), ErbB3 (ERBB3 or HER3) and ErbB4 (HER4) are involved in regulating key cellular functions (e.g. cell proliferation, survival, differentiation and migration) through a complex network of intracellular signaling pathways. Today, it is well established that abnormal expression and signaling of these receptors are associated with development and progression of several types of cancer, making them important targets for development of novel cancer therapeutics. So far, the EGFR and HER2 receptors have been the most extensively studied, with several targeting agents approved by regulatory authorities for diagnosis or treatment of human cancers. HER3 lacks tyrosine kinase activity and must therefore form heterodimers with other ErbB receptors (Citri et al, Exp Cell Res 284(1):54-65 (2003)), e.g. with HER2 to exert its signaling function. The HER2-HER3 heterodimer is considered to be a potent oncogenic unit (Holbro et al, Proc Natl Acad Sci 100(15):8933-8 (2003)) and HER3 has recently gained increased attention as the preferred heterodimerization partner for HER2. Dimerization with HER2 leads to activation of the PI3K/Akt pathway and promotion of tumor cell survival and proliferation (Hynes et al, Nat Rev Cancer 5(5):341-54 (2005)). HER3 is expressed in a number of human cancers, including breast (Bobrow et al, Eur J Cancer 33(11):1846-50 (1997)), ovarian and bladder cancer (Rajkumar et al, Clin Mol Pathol 49(4) (1996)), and play an important role in signaling in other cancers (Stove et al, Clin Exp Metastasis 21(8):665-84 (2004)), including some lung cancers (Engelman et al, Proc Natl Acad Sci 102(10):3788-93 (2005)) and prostate cancer (Gregory et al, Clin Cancer Res 11(5):1704-12 (2005)). In addition, HER3 expression has a prognostic value, since high levels of receptor expression are associated with significantly shorter survival time compared with patients that overexpress HER2 (Tanner et al, J Clin Oncol 24(26):4317-23 (2006), Reschke et al, Clin Cancer Res 14(16):5188-97 (2008)). HER3 has indeed been suggested to be a key node in ligand-induced activation of the ErbB receptor-P13K signaling axis (Schoeberl et al, Sci Signal 2(77):ra 31 (2009)).
A relatively large fraction of the more recently approved therapies directed towards the EGFR and HER2 receptors is based on monoclonal antibodies. One of the reasons behind the success for this new class of biological agents in cancer therapy is that antibodies offer new and unique mechanisms of action previously unachievable using small chemical drugs. In addition to more traditional agonistic (e.g. stimulation of a cell surface receptor) and antagonistic (e.g. blocking of natural protein-protein interactions) therapeutic effects, antibodies can also be employed as targeting agents in order to specifically direct a range of immunological defense mechanisms against the cancerous cells, as well as to achieve specific delivery of various imaging or therapeutic conjugates (e.g. chemotherapeutic drugs, radionuclides and toxins). In contrast to the well investigated EGFR and HER2 receptor members of the ErbB-family, there are relatively few reports on the use of anti-HER3 antibodies.
Ullrich and co-workers have however reported that anti-HER3 monoclonal antibodies inhibit HER3 mediated signaling in cell models of breast cancer (van der Horst et al, Int J Cancer 115(4):519-27 (2005)), and there are currently two monoclonal anti-HER3 antibodies in Phase I clinical trials (AMG 888, Baselga et al, Nat Rev Cancer 9(7):463-75 (2009), and MM-121, Schoeberl et al, supra).
However, although several successful cancer therapy studies have been reported using full-length monoclonal antibodies, this class of agents is not always optimal for targeting solid tumors (neither for diagnostic nor for therapeutic pay-load purposes). Therapeutic effect is dependent on an efficient distribution of the drug throughout the tumor, and molecular imaging depends on a high ratio between tumor uptake and surrounding normal tissue. Since tumor penetration rate (including extravasation) is negatively associated with the size of the molecule, the relatively large IgG molecule inherently has poor tissue distribution and penetration capacity. Moreover, for molecular imaging, the extraordinarily long in vivo half-life of antibodies results in relatively high blood signals and thereby relatively poor tumor-to-blood contrasts.
Since HER3 may be expressed on the same tumor cell as other members of the EGF family, production of bispecific molecules targeting HER3 and another member of the EGF family has recently attracted some interest. Such bispecific molecules could for example be utilized as targeting vehicles for increasing the specificity of targeting in molecular imaging applications and simultaneously targeting HER3 and another antigen expressed on tumors.
It is however complicated to produce bispecific monoclonal antibodies which bind to two separate targets. When the genes encoding the four required polypeptide chains are produced in a cell as many as 10 different combinations are possible, and only one combination represents the desired bispecific antibody. Therefore, the concept of bispecific binding has not been fully explored using antibodies. Instead, antibody fragments and other binding molecules have been more widely utilized for making bispecific binding molecules, including bispecific Z variants (Friedman et al, Biotechnol Appl Biochem 54:121-31 (2009)).